Pollution
Humain
Environnement
Economique

While making rounds at a plastics factory, the watchman heard a loud noise and noticed that the bracket supporting the 30″-line of the site’s aromatic unit column had fallen onto a rack. This bracket had been fastened at an 80-m height at the top of the column.

An initial inspection recorded that the bracket’s fastening bolts were corroded and sheared. Moreover, the fasteners on structural elements attached to the column that supported and guided the line were in poor condition or broken. Operating at 7 bar and 180°C for 25 years, this column had been heat-insulated. The inspection indicated that the column was indeed in a good state of repair; nonetheless, it had been slightly deformed in its vertical part and was leaning on the guides unheated.

The unit was readied for recycling before draining the column; it was shut down for repairs during 5 weeks. The column was then scaffolded over its full height; the entire line was inspected, including connection welds on the column, and the sheared bolts were examined by an expert. Bracket strength calculations were also checked.

Metallurgical and chemical analyses revealed a medium quality of the bolt steel, exposing a non-homogeneous hardness and high sulphur content. Hydrogen attack was suspected, given both the observed potential for water retention and brittle type of rupture surface. High sulphur concentration was also recorded on other bolts used on other devices around the column. This concentration did not depend on the extraction point relative to the column’s closest furnace stack (as the furnace smoke contained sulphurous by-products).

Inspections conducted on this column and others in the vicinity revealed several possibilities: a one-time obstruction of line dilatation in the guide pads, or else a weakening of metal due to the effect of higher transition temperature for bolts fastening the bracket, thereby increasing the risk of failure.

The poor quality of these bolts explained their corrosion; they were replaced by bolts of suitable quality during bracket repairs. To prevent recurrence of this type of incident, it proved useful to schedule an inspection of the supports and fasteners followed by the replacement of any defective components.